ABSTRACT The lack of biosensors fundamentally limits the reach of engineering into biology. The need to sense and measure biochemicals is central to applications spanning healthcare, biotechnology, drug development, environmental management, agriculture, food and water safety, consumer technology. Hormone measurements, in particular, are essential to fertility planning and assisted reproductive technologies; one million women in the U.S. suffer from infertility and another 7.5 million women suffer from decreased fertility. Quantitative measures of hormone levels are also critical to the diagnosis, management, and treatment of endocrine disorders and hormone imbalances that affect up to 20% of the population. Real-time, low-cost, and portable monitoring of hormones levels would enable fundamentally new approaches to fertility planning and the management of endocrine disorders. A critical limiting factor is the lack of adequate hormone biosensors. Bacteria have evolved over 3 billion years to detect and respond to virtually all classes of stimuli relevant to our own biology, including hormones. We have developed an approach to identify and isolate bacterial sensing proteins that recognize a target analyte and to develop these proteins into in vitro biosensors. We have used this approach to develop a novel hormone sensor for progesterone and we have demonstrated that this sensor is reproducible, stable, unaffected by interferents in urine, and can provide real-time hormone measurements using an inexpensive and portable electronic reader consistent with requirements for a point-of-care device. Our results thus provide proof-of-concept for a novel class of hormone biosensors adaptable to inexpensive realtime, point-of-care or consumer health devices. We will apply our approach to develop a novel class of clinically and commercially viable steroid hormone biosensors for applications in fertility planning, assisted reproductive technologies, and endocrine disorders. We will demonstrate the utility of our novel sensors by developing a point-of-care microfluidic device for real time hormone measurements from urine.